1. Field of the Invention
The present invention relates to a device, (which will be designated as a superconducting magnet device, magnetizing device and method for superconductor thereafter) for allowing a superconductor to capture an magnetic field so as to magnetize the superconductor in such an instrument as utilizes the superconductor as a magnet.
2. Description of the Prior Art
The case of making it possible to use a superconductor as a magnet is limited to only the time when the superconductor is cooled down to a superconducting transition temperature or below to bring the superconductor into its superconducting state. Therefore, it is to be understood that the superconductor, unlike a permanent magnet in general, cannot be magnetized only by applying a magnetic field thereon at a room temperature. For allowing the superconductor to capture the magnetic field so as to magnetize the superconductor (which will be designated as a magnetization thereafter), either of the following two methods will be used.
One of them is a method for magnetizing the superconductor by applying a magnetic field on the superconductor, and then cooling the superconductor down to its own superconducting transition temperature or below while keeping it applied with the magnetic field thereon. (The first method will be designated as FC thereafter).
The other is a method for magnetizing the superconductor by cooling the superconductor down to its own superconducting transition temperature or below and then applying the magnetic field thereon while holding this cooling condition. (The second method will be designated as ZFC thereafter.)
When the superconductor is magnetized, let us consider the ratio of the strength of the magnetic field (which will be designated as a trapped magnetic field thereafter) captured by the superconductor to the strength of the applied magnetic field employed for magnetizing the superconductor. Apparently, when the superconductor is magnetized in an ideal form, the ratio reaches its theoretical maximum value, which is unity in the case of FC and half in the case of ZFC, respectively. The ratio changes depending on the characteristics of the superconductors, but it never becomes a higher value than this theoretical maximum value. Therefore, it is necessary to apply a large applied magnetic field in comparison with that of the target trapped magnetic field when the superconductor is actually magnetized.
Now, in the prior art, either superconducting electromagnets or normal conductive electromagnets were used when superconductors are magnetized. In these devices, they were extremely larger in size than the superconductors to be magnetized, in the case of each having such a capacity as to generate the magnetic field necessary to magnetize the superconductors. So, in the case of integrating a superconductor into an instrument so as to be used as a magnet, the superconductor had to be integrated into the instrument after magnetizing by means of these devices at the outside of the instrument.
On the other hand, in the case of magnetizing permanent magnets, there is employed the magnetic field for generating such a current passing in one direction of a coil for a short period of time. (Such a current, a power source for generating the current and a magnetic field generated when the current is passed through the coil, respectively, will be designated as a pulse current, pulse power source and a pulse magnetic field thereafter.)
When a permanent magnet is integrated into the internal portion of an instrument and the like, particularly, in the case of magnetizing the permanent magnet by the device which employs the pulse power source, a method for integrating the permanent magnet after being magnetized into the internal portion of the instrument, is disclosed in Japanese Pat. Laid Open Pub. No.1-310516, and the other method for altering a material provided in an instrument into a permanent magnet by magnetizing it from the outside of the instrument, is also disclosed in Japanese Pat. Laid Open Pub. No.2-219440. However, since the superconductors require a refrigerant container for cooling the superconductor by refrigerant below their superconducting transition temperature, it is difficult to magnetize the superconductor from the outside of the instrument due to the long distance from the magnetizing york to the superconductor. In the case of magnetizing the superconductor by means of such a method, it becomes impossible to freely design the arrangement of the superconductor within the instrument.
As a magnetizing method of permanent magnets, there is a well-known method for magnetizing the magnetic material within the instrument by winding magnetizing coils around a material to be magnetized and incorporating all of the magnetizing coils in the instrument, and this method is disclosed in Japanese Pat. Laid Open 4-75449. In the case of the superconductors enabling to get a strong magnetic force thereby, however, the applied magnetic field required for magnetizing them becomes far large in comparison with those of permanent magnets. Therefore, the prior art magnetizing coils, which have been employed for magnetizing the permanent magnet in the internal portion of the instrument, cannot adequately magnetize the superconductor since enough magnetic field required for magnetizing the superconductor can not be obtained any more.
Ultimately, in the case of using the superconductor as a magnet by magnetizing the superconductor by means of a pulse magnetizing device of the prior art for permanent magnets, the superconductor had to be magnetized at the outside of the instrument in similarity to the case when magnetized by an electromagnet using a steady-state current. Since the pulse magnetization becomes ZFC which is disadvantageous to magnetization in comparison with FC, such a magnetizing method has hardly been employed in the case of magnetizing the superconductor at the outside of the instrument.
As described above, it is necessary to incorporate the magnetized superconductor in the instrument after magnetizing the superconductor at the outside of the instrument, in the case of magnetizing the superconductor used in the instrument as a magnet by means of the prior art magnetizing device. It is necessary, however, to maintain the superconductor at the temperature when magnetized or below in order to keep the trapped magnetic field thereof. Once the superconductor reaches higher temperatures than its superconducting transition temperature, its magnetization is completely demagnetized. As a result, since it is necessary to keep the internal portion of the instrument within a refrigerant, it will be very difficult to incorporate the superconductor magnetized at the outside of the instrument into the internal portion of the instrument while keeping the superconductor at the temperatures which could keep its trapped magnetic field as it is.
Then, in order to keep the trapped magnetic field of the superconductor incorporated into the instrument, it is also necessary to keep the superconductor cool by supplying refrigerant at all the time and it may take very long time whether the instrument may be operated or not.
Furthermore, even though the superconductor may be magnetized once, the resulting magnetic flux thereon creep with time and the trapped magnetic field is weakened. Therefore, for preserving the capacity as a magnet, it is necessary to take out the used superconductor from the instrument after a certain period of time and then magnetize the superconductor again. Carrying out such an operation is very difficult due to the cooling problem described above.